1. Field of the invention
The present invention relates to a process for producing synthetic video images with a view to a real time visual display with a high information density. The invention also relates to any apparatus using this process. The invention is more particularly intended for use on board aircraft in navigation aid systems utilizing the visual display of symbols on cathode ray tube screens.
2. Prior art
There are various types of synthetic image generators having different performance levels. Among the most efficient, random symbol generators supply sombols in accordance with a random scan. These symbol generators are fast, work in real time, have a good integration and a highly developed software, however, figuring is linear. The other types of generators, such as the commercially available LSI, CAO systems and simulations suffer from disadvantages in certain applications, such as poor representation and large overall dimensions. The hitherto known aircraft-installed synthetic image generators use the random scanning mode the television scan generally being reserved for video image sensors, such as cameras, FLIR, etc.
The object of the invention is to provide a random image generator which, as a result of an image memory, can control visual displays using scans which can be of different types, namely a random scan for which the image generator is provided in a direct link, or other scan types via an image memory, such as framed or so-called television scanning, or matrix scanning.
In the case of an outline by random scanning in real time, the symbol generator has to make the outline again, even if certain figures are fixed or revolve only slowly. The outline is repeated at a frequency compatible with the retinal persistance. For a given scan period corresponding to the duration of an image outline, the capacity of the outline is limited, in view of the fact that for each figure or element which is variable, the calculator has to carry out calculations and indicate at each period the new outline data to the symbol generator. Thus, as the number of figures to be drawn increases, saturation is reached or it is necessary to increase the duration of image drawing. However, this duration cannot exceed the limit compatible with the notion of retinal persistance, i.e. approximately 25 images per second.
In order to obtain a graphic image according to a television scan, it is known to use an image memory and to work according to the so-called double page process, i.e. a first image memory is used on writing, whilst a second image memory is scanned on reading and erased as said reading is carried out and so on, whilst switching the image memories at the end of each reading operation. This solution is disadvantageous in that the memory which is read for the display is erased as matters progress, and all the elements of this memory must then be refreshed by the symbol generator.
According to the invention, the distinction is made between the fixed or slow evolution symbols, which to a certain extent constitute the "decor" and the faster evolution symbols which constitute what is called the "actors". It is proposed to place the decor in one or more image plane. The term image plane is understood to mean a memory means containing all the informations producing an image. The actors are located in one or more other image planes. It is necessary to modify the position of the fast evolution symbols, i.e. the actors as often as possible. Thus, this modification takes place in the timing of the video, i.e. at the image reading rate. However, it is not necessary to modify as frequently the image plane or planes containing the fixed or slow evolution elements constituting the decor, the corresponding refreshing taking place over a longer period. Thus, for each video cycle of reading an image, it is necessary to modify the image plane containing the actors, whilst that containing the decor is only modified during certain cycles or over a certain number of cycles.
The proposed image generator is consequently constituted by a random symbol generating means, specific adaptors, interface circuits, etc., as well as image memories.
This synthetic image generating means has numerous advantages. It makes use of an image generator, e.g. of the VLSI type, which has high performance levels, permits a high volume capacity of graphic descriptions, and has a low consumption level. It also uses all the basic software (assemblers, simulators, use for both framed and random applications). It makes it possible to display framed images, with all the flexibility of random symbol generators (rotation, circle, etc.). The same symbol generator can control in parallel directly the display on screen where the drawing takes place with random scanning, as well as indirectly via image memories, the display on cathode ray tube screens where scanning takes place in framed form. On a television image from a sensor, it makes it possible for "incrustation" to take place with all the flexibility of existing random symbol generators. It uses the same operational software for presenting in certain cases the same images on screens of different types. The figuring normally provided on a display in the random mode can be presented on another display in the framed mode without requiring the program of two different symbol generators. It facilitates reconfiguration in the case of a breakdown.